The invention relates to a control circuit forming part of plasma jet power equipment that can be used for transferred arc cutting and welding duties in conjunction with a torch designed basically for non-transferred arc operation.
The prior art embraces designs wherein a cylindrical electrode is attached rigidly to the plasma torch and connected to the negative pole of the electrical power supply by a conductor. In such instances, the anode takes the form of a cap, likewise rigidly attached to the torch, which covers the projecting tip of the electrode. The cap is insulated from the electrode, and connects with the positive pole of the power supply via a second conductor, a fine gap being created between cap and electrode through which plasma-forming gas is directed under pressure toward an orifice in the cap itself; the pierced cap thus constitutes a constricting nozzle. In a first conventional type of torch, an arc is struck between nozzle and electrode utilizing a particularly high auxiliary voltage, which in most instances will be generated by complex, costly high frequency circuitry to which the torch is connected by long, heavily insulated cable.
What occurs, in practice, is that one generates a spark between the electrode and the nozzle, which occupy fixed positions in relation to one another; an arc is struck, and continues to discharge until such time as the torch push button or power supply switch is shut off. Accordingly, the arc is struck between components of the torch, and not transferred to the work.
The expedient widely adopted to permit transferred arc operation of such a torch is that of connecting the work (welding or cutting) to the same positive terminal as that to which the nozzle return lead is connected, and wiring-in a resistance between the return lead and the terminal. Thus, when the torch is moved toward the weld or cut, the arc encounters less resistance from the work than from the return lead, and jumps across to the work.
The same effect can be produced by wiring-in switch contacts, rather than a resistance, which will be broken when the weld or cut is commenced.
In a second conventional type of torch, designed for transferred arc operation, the arc is struck between the electrode and the work; the electrode is rigidly attached to and shielded by the torch and connected to the negative terminal of the power supply, as before, though in this instance it is the work to be welded or cut, and not the nozzle, that is connected to the positive terminal.
The cap, or nozzle, is no longer rigidly attached in this type of torch, and an arc is struck by bringing the nozzle into momentary contact with the electrode and the work, sandwiching it swiftly between the two and then releasing. This physical contact between the electrode, nozzle and work short-circuits power through the torch, whereupon, with the electrode distanced from the nozzle, the gap created between electrode and nozzle produces an arc, and the arc is transferred from the electrode to the work.
It will be appreciated that good initial contact between nozzle and work is a fundamental requirement in plasma welding or cutting with a transferred arc; this in turn dictates that the work must offer a sufficiently clean surface, so that a certain amount of preparation is called for, in particular where there may be rust, old paintwork. Transferred arc equipment must therefore incorporate special safety circuits to prevent high voltage from reaching the electrode unless the electrode itself is faultlessly positioned for operation, that is, in contact with the work; without such a precaution, the high welding voltage would invest the operator, who will normally be in physical contact with the work.
Such a safety feature is set forth in EP No. 159256, which discloses the interposition of a low voltage circuit between the electrode and the work to be welded or cut, or rather, between the two relative terminals, the output signal from which operates a control medium designed to switch the main high voltage circuit into safe condition.
More recently, the same applicant has developed a non-transferred arc torch design in which the fixed electrode (wired to negative as before) is retained, whereas the nozzle (wired to positive) is capable of movement in relation to the electrode between a transitory position, in which contact is occasioned so as to strike the arc (the arc being struck, in reality, immediately following subsequent separation of the two components), and the normal operating position in which the nozzle is distanced from the electrode; once the arc is struck in such a torch, it will continue discharging, non-transferred, until such time as the control button on the grip of the torch is released to break the power circuit.
This design of torch solves the problem of excessive cost besetting the high frequency equipment referred to above, but remains subject to the usual drawbacks of non-transferred arc operation--i.e. a less than perfectly clean cut, especially around the edges, and overheating of the torch itself, and in particular the nozzle, which thus wears out that much earlier.
The object of the invention is that of embodying a control circuit for transferred arc type plasma jet cutting or welding equipment that can be used for both non-transferred and transferred arc operation, thereby gaining the advantages relative to either method.
A further object of the invention is to embody a circuit that will allow of striking an arc initially between electrode and nozzle, and of transferring the arc to the work thereafter as and when required, even in cases where the surface of the work may be rusted, or have undergone treatment at some previous time.
Another object of the invention is that of embodying a circuit which allows a non-transferred arc to be struck in the torch without using high frequency power and enables transferred arc operation of the torch, whilst ensuring the personal safety of the operator in either instance, especially the latter instance.